In the past few years, simple architectures for the photonic generation of radio-frequency, RF, signals have been envisioned based on the heterodyning of two independent lasers, but these implementations do not allow for stable RF generation, meaning that the resulting signal is not useful for the requirements of future systems. In order to improve RF signal stability, phase locking of the heterodyned lasers is necessary, and this usually requires more complex and cumbersome set-ups. A relatively simple technique for generating phase locked laser lines is the mode locking of lasers; the intrinsic phase-locking condition of the mode-locked laser, MLL, ensures that the generated RF signal has extremely low phase noise. Moreover, the possibility of selecting laser modes with variable wavelength detuning allows the flexible production of RF carrier signals with tuneable frequency, potentially generating at any multiple frequency of the MLL repetition rate. Moreover, the phase noise of the resulting RF carrier signals has been demonstrated to be significantly less noisy than RF signals generated using state-of-the-art RF synthesizers.
A recent approach for generating stable RF clock signals is the use of an opto-electronic oscillator. OEO. An OEO relies on modulating the light from a laser and feeding back the generated RF signal to the input port of the modulator itself. OEOs have been shown to have extremely low phase noise, thanks to the ultra-low losses of the optical fibres from which they are constructed. An OEO therefore acts as a resonator with an extremely high finesse, i.e., with the capability of storing large energies for a long time.
The first examples of OEOs made use of long fibre delay lines and RF filters in the feedback to fix the oscillation frequency. This approach has the drawback of not allowing frequency tuning of the generated RF signal. Another OEO implementation substitutes the fibre delay line and the RF filter with an optical resonator, enabling wide RF frequency tuning to be achieved. However, in this implementation some of the optical power after the modulator is lost in the filtering operation, which selects a single sideband from the two original sidebands. Therefore, the loss of the feedback loop is increased, and this induces higher phase noise in the oscillator signal.
More recently, OEOs based on phase modulation and phase-to-amplitude modulation conversion have been proposed. The use of a phase modulator instead of an amplitude modulator reduces the insertion losses of the OEO. Moreover, phase modulators do not require a bias voltage for setting their working point, simplifying the operation. In one OEO implementation the phase-modulation to amplitude-modulation conversion is realized by filtering out one of the first-order sidebands generated by the phase modulation. In this way, the phase modulation is transformed into a single sideband amplitude modulation, which can be detected by the photodiode to close the opto-electronic feedback loop. However, even in this case the filtering operation cancels part of the optical power, worsening the phase noise of the oscillation signal.
W. Li, J. Yao, “A wideband frequency tunable optoelectronic oscillator incorporating a tunable microwave photonic filter based on phase-modulation to intensity-modulation conversion using a phase-shifted fiber Bragg grating”, IEEE Transactions on Microwave Theory and Techniques vol. 60, no. 6, 2012, reports the conversion from phase-modulation to amplitude-modulation using a phase-shifted fibre Bragg grating, PS-FBG, in reflection. A fibre Bragg grating, FBG, is a distributed Bragg reflector formed within a short section of optical fibre that reflects wavelengths of light that meet the Bragg condition and transmits all other wavelengths. A PS-FBG is an FBG which has a phase-shift generally at the centre of the structure, which results in a transmission band within the reflection spectrum of the FBG. In the OEO reported by Li and Yao an PS-FBG is used in reflection and is configured so that one of the side bands generated by the phase modulation falls within the transmission notch of the PS-FBG and thus has a π phase-shift applied to it. In this way the optical power of the affected side band is not cancelled, so the phase modulation is transformed into a double sideband amplitude modulation. Since the resonance frequency of the PS-FBG is not tuneable, in order to change the oscillation frequency of the OEO a tuneable laser is required, which implies a higher cost. Also, the phase shift of the PS-FBG is associated with a notch in the amplitude response, which while not cancelling the affected side band does reduce its optical power, therefore worsening the phase noise of the generated RF signal.